In order to decrease the amount of the emission of nitrogen oxide (NO.sub.x) component in an internal combustion engine, an exhaust gas recirculation (EGR) apparatus is used. In this apparatus, a part of the exhaust gas is recirculated from the engine exhaust system to the engine intake system in order to decrease the combustion temperature so that the amount of NO.sub.x component in the resultant exhaust gas is reduced.
If the EGR operation is carried out when the engine is operating under a warm-up condition in which the temperature of the engine has not yet sufficiently increased, an adverse effect will inevitably occur thus preventing a proper engine operation. Therefore, for a vehicle of a relatively light weight, it is preferable to provide means for stopping the EGR operation when the engine is operating under a warm-up condition.
However, when the EGR apparatus is utilized in a heavyweight vehicle such as a truck, the EGR operation should not be completely stopped during the warm-up condition. Because, if the EGR operation is not carried out, a large amount of NO.sub.x is exhausted from the heavyweight vehicle, which excessive amount is not allowed by the present strict legal restriction limiting the amount of toxic components that can be emitted from an engine.
However, if an EGR operation exhibiting a large EGR ratio is carried out during the warm-up condition, it is necessary to step very hard on the accelerator pedal in order to obtain a large throttle opening for achieving a sufficient output power. This is because during the warm-up operation as well as during the EGR operation, a relatively large amount of fuel is required for operating the engine. Due to the large throttle opening, the amount of intake gas sucked into the engine becomes very large, therefore, large amounts of hydrocarbon (HC) and carbon monoxide (CO) components remain in the exhaust gas. These toxic components cannot be effectively cleaned by a catalytic converter arranged in the exhaust pipe, because in this warm-up condition the temperature of the exhaust gas cannot reach the proper temperature at which an effective cleaning operation of the CO and HC components performed by the catalytic converter can be obtained.